Many conventional optical components incorporate a photosensitive cell. For example, optical detectors typically include a single element, or cell, sensitive to the light incident on its entry face. As another example, optical detectors may be formed from a matrix of photosensitive cells that are juxtaposed alongside another. Thus, the respective entry faces lie in a common plane. The photosensitive cells are generally of small dimensions in order to obtain a sufficient spatial resolution. For example, the light entry face may have dimensions of approximately 5 μm×5 μm. Consequently, the quantity of light entering the photosensitive cell is limited by the size of the entry face.
The cells detect the light with a photosensitive element which may be based, for example, on CMOS technology. Thus, it is possible to convert the received light signal into an electrical signal. The photosensitive elements require at least one and often three metallization levels in order to connect them to the circuits that control them or that process their information. The metallization levels are formed from tracks that are generally made of copper and are isolated by dielectric layers. The metallization levels are formed between the photosensitive element and the light entry face of the cell. The dielectric materials used for isolating the tracks must therefore be transparent at the wavelengths detected by the photosensitive element. The light, however, is attenuated between the entry face and the photosensitive element especially when the number of metallization levels grouped into a multilayer increases.
To limit this attenuation, conventional methods produce an optical guide passing through the multilayer. In order to accomplish this, each multilayer level has a via. The vias are aligned and filled with a material whose refractive index is higher than the refractive indices of the materials forming the multilayer. It is, however, tricky to produce such a guide since it requires strict alignment of the constituent vias. If the vias are not aligned properly the guide could allow the photons initially propagating therein to escape. In addition, the photosensitive element is generally a surface portion of the semiconductor substrate, said surface portion being especially treated so as to fulfil this function. However, the surface of the substrate also includes, inside the cell, apart from the photosensitive element, electronic components needed for the operation of the photosensitive element.
The photosensitive element possesses a photosensitive surface of smaller area than the area of the entry face. For example, the dimensions of the photosensitive surface are 2 μm×2 μm for an entry face measuring 5 μm×5 μm. Therefore, incident light in the form of a uniform beam directed approximately perpendicular to the entry face, and which illuminates the semiconductor substrate at the electronic components is lost. Thus, the light does not reach the photosensitive element and is substantially that part of the light which does not return to the optical guide. This loss reduces the sensitivity of the cell, particularly for low incident light intensities, compared with the theoretical sensitivity defined by the size of the entry face for light to enter the cell.
Some conventional methods provide a light guide by removing a part of the interconnection dielectric material and by depositing a material of high refractive index. This light guide has a frustoconical shape, the large cross section of which is located near the entry face and the small cross section of which is located near the photosensitive element. The element forming a light guide has the effect of concentrating the light entering via the large cross section onto the smaller surface of the photosensitive element. The production of such light guides involves the formation of a hole with a depth very much greater than its diameter, followed by the hole being filled with a material of high refractive index. This introduces, when filling the guide defects may cause light losses. In addition, as these defects are more numerous, the larger the height of the light guide.
Therefore, what is needed is an optical coupling element of high efficiency, whose efficiency depends little on the alignment defects of the various parts of the integrated circuit.